The long-range objective of this program is to determine the behavioral and physiological processes that contribute to differences between two species of South American primates in social organization and relations to the natural environment. One species (Callicebus, titi monkey) is monogamous and territorial; the other species (Saimiri, squirrel monkey) lives in large sexually segregated groups and occupies an extensive undefended home range. Male titis are actively involved in care of offspring, whereas male squirrel monkeys are not. Current studies with Callicebus have emphasized family dynamics. The behavioral and physiological responses of male and female Callicebus to their own and to alien infants and juveniles have been examined. Both parents responded more vigorously to infants than to older immatures, but did not distinguish between their own and alien offspring. In contrast to all other mammals for which comparable data are available, Callicebus infants prefer their fathers and respond more vigorously to separation from them than to separation from their mothers. Immature and mature offspring living in their natal groups do not participate in the care of infant siblings. Developmental changes in responses of offspring to strangers and to parents have been examined in several studies. Findings suggest that attraction to parents persists at a high level into reproductive maturity, even though responsiveness to strangers increases. The bond between adult male and female is the subject of other research. Behavioral and physiological responses to varying periods of separation from the pairmate and effects of separation on responses to strangers and to reunion with the pairmate have been examined. Behavioral and physiological responses to the formation of pair bonds are being investigated. Callicebus and Saimiri have been compared in their responses to novelty. In keeping with their cautious approach to the world, Callicebus are more responsive than Saimiri to environmental change. *KEY* Territoriality, Monogamy